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The technical information for some bicycle bottles includes statements like:

Do not microwave. Do not add fatty products (oil, milk, etc.).

Not microwaving, I understand. Not putting in a dishwasher, I also understand.

But what is the reason for not using for milk, etc? Hard to clean? Damages the bottle? Reacts to produce toxins?

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    I dunno about your tastes, but milk would go stank pretty quick on a ride and be undrinkable to me. That includes milk-based drinks like iced coffee.
    – Criggie
    Commented Feb 12, 2022 at 5:41
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    In the case of milk you may find you have churned it into butter after several miles...
    – PCARR
    Commented Feb 12, 2022 at 15:07
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    @Criggie Only times I've taken anything milk-based it was fine after 20 miles (chocolate milk in its original plastic bottle). But ambient was around 2degC...
    – jhnc
    Commented Feb 12, 2022 at 16:35
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    @PCARR My intestines would be churned into butter if I tried drinking milk on a ride.
    – MaplePanda
    Commented Feb 13, 2022 at 4:50
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    @PCARR mmmm fresh butter... can also work as a last-resort chain lube !
    – Criggie
    Commented Feb 16, 2022 at 10:27

3 Answers 3

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This is a very simplified explanation, but plastics are long chain polymers. Fats are also longish chain polymers and, depending exactly on the plastic, they can "stick together" (the chains have little bits that stick out, and they can grab or hold on to each other). You may have noticed that some plastics have a "waxy" or "greasy" feel, and that it can be harder to clean oil or grease from a plastic container than from, say, glass or metal. So, depending on the exact type of plastic for water bottles, you don't want to put fatty liquids in the bottle not because it harms the bottle or harms the liquid or creates an immediate health or safety issue: you don't want to put them in the bottle because it's hard to clean, which could cause a subsequent issue. If you get a grease mark on the outside of your water bottle, is it hard to clean off? It's the same on the inside of the water bottle.

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    What happens is the fatty acid actually bonds to the plastic very strongly, and it is a stronger bond than diluted dish detergent. In order to clean properly, you need to use completely undiluted detergent directly onto the oily part, and may need to do it multiple times. The moment you dilute it, the grease is not going to get released from the plastic. Most people simply don't know how to clean their bottles this way and don't clean their bottles properly.
    – Nelson
    Commented Feb 12, 2022 at 13:39
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    +1 for the main point, but “not because it […] creates a health and safety issue” seems a bit inaccurate — using fatty liquits will most likely lead to the bottle staying not-quite-clean between uses, which can definitely become a health and safety issue.
    – PLL
    Commented Feb 12, 2022 at 18:11
  • @PLL I think the important part of that sentence is the word immediate. Maybe a better phrasing would be direct vs indirect - the fatty liquid doesn't directly cause a health or safety issue, but it makes it harder to clean, which indirectly causes potential health and safety issues.
    – Cullub
    Commented Feb 14, 2022 at 18:41
  • Thanks so much for this, it explains why my plastic lunchboxes feel greasy even after I've washed them if I've had an olive oil vinaigrette or something in them - it's because they are!
    – Wilskt
    Commented Feb 16, 2022 at 10:43
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I cannot find any references to milk damaging any form of plastic in a way that makes it toxic; without knowing the plastic used (what's the recycle number on the bottle?), it's hard to do more research. Milk in many countries is distributed in plastic bottles, and baby bottles these days are mostly plastic, so plastic than can handle milk products safely is presumably available. I believe it is incredibly unlikely the reason is increased toxicity over plain water (I put it this way to avoid debate of things like BPA and carcinogens leaching from plastic bottles).

Cleaning oils and fatty residue from the likes of milk requires fairly high temperatures. Microwaving also has the potential to generate high temperatures, and dishwashers can generate very high temperatures in the drying cycle. Its likely the manufacturer does not want the bottles used with these liquids because they cannot be cleaned without risking damaging the bottle.

If they are not cleaned properly, these liquids go, how to put it, rank, and ruin the bottle.

So by putting these liquids in the bottle, you risk ruining it by cleaning it properly, or ruining it by not cleaning it properly.

PET, a common plastic for food products, cannot handle high temperatures and leaches Antimony at high temperatures, therefore it's likely the bottle being asked about is PET (Recycle #1).

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  • The particular bottles I have are from decathlon and marked #5 PP
    – jhnc
    Commented Feb 12, 2022 at 16:41
  • @jhnc Sadly those markings are not terribly useful - the main ingredient is polypropylene, but for health issues it's more important which additional unlabelled ingredients are added in small quantities.
    – Nobody
    Commented Feb 12, 2022 at 19:29
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    A small of amount of antimony is leached from PET bottles (it's added as antimony trioxide (Sb2O3) as a catalyst) and it leaching is enhanced more by citrates in fruit juices than my heat. pubmed.ncbi.nlm.nih.gov/32739689 The EU limits concentration of antimony in packaged goods to 5 μg/L (Commission Directive 2003/40/EC), and I understand that the limit in the United States is 6 μg/L
    – D Duck
    Commented Feb 12, 2022 at 20:27
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Unfortunately, creating an all-foods-safe container is a challenge.

The only substance one can always rely on is the glass. The food grade glass - sorry, there are other types that are not food-safe. ... and then, if you need the glass container closed, you get to the sealant. There are (profoundly impractical) glass sealants. You don't want them in a consumer product.

All other substances CAN interact with one type food or another.

  • Linear polymers mutually dissolve with fats.
  • All polymers leech production process artifacts (catalyst, monomers, oligomers), dyes, contaminants, fillers, etc... depending on the substance they are in contact with, the mechanical deformations, age and other factors. Here goes e.g. our beloved bis-phenol-A.
  • Metals dissolve ions in acids normally present in foods.
  • So do some ceramic coatings
  • ...and some natural stones (up to and including full dissolution).
  • Wood lets some water and some alcohol out and dissolves a lot of its constituents. etc, etc, etc...

In particular, it is quite hard to create:

  • Multiple use (e.g. years of partial deformations that promote migration of unwanted substances from the bulk of the material to the surface)
  • Soft enough not to crack or shatter into pieces
  • Safe to be in contact with drinking water (but not milk or wine)
  • Reasonably resistant to UV light
  • Reasonably priced

Up to these requirements, we have some degree of success.

If you start to add:

  • Resistant to food fats
  • Resistant to food acids
  • Resistant to essential oils
  • Resistant to heat (boiling water, baking oven) ...

The task becomes unsolvable without disposing off some other requirements.

e.g. one can make it resistant to alcohol and acids enough to sell cheap wine in it, but the bottle becomes single use.

...or...

one can make plastic baby food containers that can be boiled, but they are nowhere soft.

...or...

a baby pacifier - soft, heat-resistant, but of limited lifespan and expensive.

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    I, for one, will not be upgrading to glass bottles on my bike. Imagine the weight penalty !
    – Criggie
    Commented Feb 15, 2022 at 11:13
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    I think the cutting edges of the broken glass are worse than the weight.
    – fraxinus
    Commented Feb 15, 2022 at 12:32
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    @Criggie I'm questioning fraxinus' commitment to Strava segment-hunting /s
    – Swifty
    Commented Feb 16, 2022 at 7:41

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